commands: clear error for malformed decimal CLI arguments

[devdavidejesus]

Fixes #7404.

Command-line arguments typed as decimal (decimal, decimal_or_dryrun, decimal_or_max in arg_types) were converted with lambdas that raise decimal.InvalidOperation on malformed input. argparse only turns ArgumentTypeError, TypeError and ValueError from a type function into a usage error, so any malformed amount crashed with a traceback. This affects every command with a decimal argument, not only add_request (e.g. payto <address> abc).

The conversion now goes through a small helper that raises argparse.ArgumentTypeError:

$ electrum --testnet --offline add_request help
run_electrum add_request: error: argument amount: invalid decimal value: 'help'

The special values (! for max, dryrun) are handled as before. Test: test_cli_decimal_args_reject_malformed_values in tests/test_commands.py.